The recording density of hard disc drives has increased for almost three decades. Since longitudinal magnetic recording media is constrained by the thermal stability limit, perpendicular media is the most prominent candidate for the next generation media. A typical perpendicular media comprises of a multilayer structure including a substrate covered by a soft magnetic underlayer, an interlayer, and a hard magnetic layer covered by an overcoat and an organic lubricant. The hard magnetic layer is the layer on which the information is stored. The magnetic layer can be comprised of a hexagonal close packed structured (hcp-structured) Co-alloy or a magnetic multilayer, such as Co/Pt, Co/Pd, etc.
Magnetic properties of the storage layer such as coercivity (Hc), remanent magnetization (Mr), remanence squareness of the hysteresis loop, slope of hysteresis at the coercive point (α=4πdM/dH|H=Hc), and magnetic cluster size (d*), are crucial to the recording performance of the Co-alloy media. An objective in developing a perpendicular magnetic recording medium is to achieve a thermally stable medium with enhanced signal-to-noise ratio (SNR). The requirements for achieving this objective include, for example, suitable microstructure such as a well defined (00.2) orientation, small grain size, narrow grain size distribution, low amount of fcc phase, and low stacking fault density. Additional requirements include magnetic properties, such as, sufficient magnetic anisotropy (Ku), an anisotropy field (Hk) compatible with write fields, high negative nucleation field (Hn), full remanence squareness, and optimized intergranular exchange interaction. Although these requirements are similar to the requirements for other types of media, such as longitudinal media, the implementations are different due to differences in crystallographic orientation and layer construction. In a perpendicular magnetic recording medium of Co alloy material, crystalline grains are oriented with the c-axes (i.e., the magnetic easy axis) in a direction normal to the film planes. The intrinsic perpendicular anisotropy energy needs to be larger than the demagnetization energy, which typically requires a low stacking fault density.
To achieve high magnetic remanence, the orientation dispersion around the film normal should be as small as possible. To achieve a low noise medium, the average Co-alloy grains should be as small as possible but within the thermal stability limit. The grain size distribution should be as narrow as possible. Moreover, the grain boundaries should be able to magnetically isolate the neighboring grains. An interlayer which consists of the layers between the magnetic storage layer and the magnetic soft underlayer is very crucial to the microstructure of the magnetic layer.
Interlayers can effectively improve the orientation distribution, as well as enhance the epitaxial growth of the Co alloy grains. Interlayer materials which have been reported in the literature include Pt, Ge, Si, CoCr, SiO2, Au, Al, CoZr, Ta, Ti, TiCr, Ru, RuCrCo, TiZr, etc. Only a few of them work well enough to meet the demands of the industry. Of all the interlayers, RuCoCr has the best structural match and lattice match. Epitaxial growth of Co-alloy grains on top of RuCoCr has been confirmed by high-resolution electron microscopy. However, little has been done so far as to reduce the grain size of the interlayer, so as to decrease the grain size of the magnetic layer. On the other hand, it is probably impossible to obtain one single interlayer fulfilling all the aforementioned criteria to achieve the desirable microstructure, magnetic properties and recording performance.
The CoCrPtB alloy is well-known for its small grain size, narrower grain size distribution, and proper chemical segregation at the grain boundaries. Its performance for longitudinal media is superior to other alloys. However, when CoCrPtB is used for perpendicular media, it is found to have high stacking fault density, which results in poor magnetic properties. On the other hand, perpendicular media made of CoCrPt have much fewer stacking faults. Therefore CoCrPt media have full squareness, larger Hk and higher Ku. However, the magnetic grains of CoCrPt are more likely to be coupled than grains of CoCrPtB. Moreover, chemical-ordered hexagonal phase Co3Pt is found to have hard magnetic properties superior to hcp Co-alloys.
This invention provides a design that best utilizes the advantages of CoCrPtB, CoCrPt and Co3Pt media while maintaining the excellent perpendicular orientation of the magnetic layer.